The holotype specimen (GI SPS N 700/17), a complete skull, was recovered from the Bayan Shireh Formation (Cenomanian-Santonian), at the Tsagan-Teg ("White Mountain") locality, Dzun-Bayan, in the southeastern Gobi Desert, Mongolia. At this time, the genus is monotypic, including only the type species, T. longicranialis.[1]

The skull measures 30 centimetres (12 in) in length, with a maximum width of 25 centimetres (9.8 in). Vickaryous et al. (2004)[2] note that "Unlike other ankylosaurs, in Tsagantegia the cranial ornamentation is not subdivided into a mosaic of polygons but is amorphous." The quadratojugal and squamosal bosses are poorly developed, in contrast with other ankylosaurs. The skull was long and flat, smooth and bearing small horns.